The present invention relates to a method for logging formations penetrated by boreholes and particularly, a method for acoustically detecting the characteristics of formations having relatively low acoustical velocities and high acoustical losses. These formations normally comprise weak sand structures. As is well known, considerable quantities of both crude and natural gas are found in sand structures. Thus, the location and type of sand structures penetrated by a borehole is important.
In addition, the location of weak sand structures is important if they contain commercial quantities of petroleum products. If commercial quantities are located in weak sand structures and their character and location is known, steps can be taken before completing the well to stabilize the sand structures. If weak sands are not stabilized, they will tend to flow into the well when it is produced and thus, clog the well and either completely stop or diminish production. While it is possible to consolidate weak sands after the well is completed, superior results are obtained if they are consolidated prior to casing the well.
One method that has been suggested for locating weak sands in formations penetrated by a borehole is the use of an acoustical logger. The method consists of using an acoustical logger and measuring the response of the formation to compressional waves. In the presence of weak sands, the travel time of the compressional waves is increased due to the lower acoustical velocity of the sands. The increased travel time can be used as an indication of weak sands. While this approach has some success, it often times fails since the decrease in velocity of the compressional waves in weak sands is usually small and affected by other variables such as the presence of clay.
In an attempt to overcome the problems of conventional acoustic loggers it has been suggested that the velocity of shear waves be measured. This has some success where formation velocities are high, i.e., greater than 10,000 feet per second. For lower velocity formations the shear wave event is obscured by the earlier arriving compressional waves. In cases where the shear wave velocity in the formation is exceeded by the compressional wave velocity through the borehole fluid the method fails completely. This failure is the result of the inability to generate shear waves in the formation under these conditions using conventional methods.
Some success has also been obtained by measuring shear wave velocities in core samples. This requires expensive coring operations to obtain the cores since low velocity formations are really incompetent and tend to fall apart when cored. Further problems arise when measurements of shear wave velocities are made since it is difficult to couple the transducers to the core.